Endoluminal stent having mid-strut interconnecting members

ABSTRACT

An endoluminal stent composed of a plurality of circumferential expansion elements arrayed to form the circumference of the stent and extending along the longitudinal axis of the stent, and a plurality of interconnecting members that interconnect adjacent pairs of circumferential expansion elements, the interconnecting members joining struts of adjacent pairs of interconnecting members at approximate mid-points of the struts.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority to currently pending U.S.Provisional Application Ser. No. 60/455,783 filed Mar. 19, 2003.

BACKGROUND OF THE INVENTION

The present invention relates generally to endoluminal stents, coveredstents and stent-grafts designed for delivery into an anatomicalpassageway using minimally invasive techniques, such as percutaneousintravascular delivery using a delivery catheter passed over aguidewire. More particularly, the present invention relates toendoluminal stents having a scaffold structure and structural geometrywhich is particularly well-suited for providing physiologicallyacceptable radial or hoop strength and longitudinal flexibility, whilealso presenting a luminal surface thereof which presents lessobstruction to longitudinal shear forces during fluid flow across theluminal surface of the inventive device while maximizing fatigue lifeand corrosion resistance. Additionally, the inventive endoluminal stentis characterized by a geometry that uniquely has a negative coefficientof longitudinal foreshortening upon radial expansion. Thus, a uniqueaspect of the inventive endoluminal stent is that it elongates uponradial expansion.

Endoluminal stents are generally tubular scaffolds fabricated fromimplantable biocompatible materials. Stents have a generally tubulargeometry characterized by a central lumen, a longitudinal axis, acircumferential axis and a radial axis. Conventional endoluminal stentsfall within three general classifications: balloon expandable,self-expanding and shape-memory. Balloon expandable stents requiremechanical intervention, such as by using a balloon catheter, to apply apositive pressure radially outward from a central lumen of the stent tomechanically deform the stent and urge it to a larger diameter.Self-expanding stents utilize inherent material mechanical properties ofthe stent material to expand the stent. Typically, self-expanding stentsare fabricated of materials that rebound when a positive pressure isexerted against the material. Self-expanding stents are fabricated suchthat their zero-stress configuration conforms to the second largerdiameter. The self-expanding stents are drawn down to the first smallerdiameter and constrained within a delivery catheter for endoluminaldelivery. Removal of the constraint releases the constraining pressureand the self-expanding stent, under its own mechanical properties,rebounds to the second larger diameter. Finally, shape-memory stentsrely upon unique alloys that exhibit shape memory under certain thermalconditions. Conventional shape-memory stents are typicallynickel-titanium alloys known generically as nitinol, which have atransition phase at or near normal body temperature, i.e., 37 degreesCentigrade.

The prior art is replete with various stent designs across all stentclassifications. One of the difficulties with many conventional stentdesigns arises due to the conflicting criteria between the desiredproperties of circumferential or hoop strength of the stent,longitudinal or column strength, longitudinal flexibility, fish-scalingof individual structural members of the stent, fatigue life, corrosionresistance, corrosion fatigue, hemodynamics, radioopacity andbiocompatibility and the capability of passing the stent through analready implanted stent. Typically, stents that are designed to optimizefor hoop strength typically sacrifice either column strength and/orlongitudinal flexibility, while stents that are designed to optimize forcolumn strength often compromise longitudinal flexibility and/or hoopstrength.

Most conventional stents exhibit longitudinal foreshortening upon radialexpansion of the stent. Longitudinal foreshortening is a well-knownproperty that results from the geometric deformation of the stent'sstructural members as the stent radially expands from a contracted stateto a diametrically expanded state. Several prior art stents have beeninvented that claim a lack of appreciable foreshortening of the stent asa novel feature of the stent. Heretofore, however, a stent thatlongitudinally elongates upon radial expansion from a contracted stateto a diametrically expanded state is unknown in the art.

It has been found desirable to devise an endoluminal stent which employsa series of first and interconnecting members arrayed in geometricalpatterns which achieve a balance between hoop strength, column strengthand longitudinal flexibility of the endoluminal stent. Many conventionalstents employ a series of circumferential structural elements andlongitudinal structural elements of varying configurations. A largenumber of conventional stents utilize circumferential structuralelements configured into a serpentine configuration or a zig-zagconfiguration. The reason underlying this configuration is the need forradial expansion of the stent. Of these conventional stents which employserpentine or zig-zag circumferential structural elements, many alsoemploy longitudinal structural elements which join adjacentcircumferential structural elements and provide a modicum oflongitudinal or column strength while retaining longitudinal flexibilityof the device. Additionally, many conventional stents require welds tojoin mating surfaces of the stent.

Heretofore, however, the art has not devised a unibody stent structuralelement geometry which achieves a balance between hoop strength, columnstrength and longitudinal flexibility, degree of longitudinalforeshortening, circumferential strength or hoop strength of the stent,longitudinal strength or column strength, longitudinal flexibility,fish-scaling of individual structural members of the stent, fatiguelife, corrosion resistance, corrosion fatigue, hemodynamics,radioopacity, biocompatibility and the capability of passing the stentthrough an already implanted stent. The term “fish-scaling” is used inthe art and herein to describe a condition where some stent structuralelements extend beyond the circumferential plane of the stent duringeither radial expansion, implantation or while passing the stent througha bend in the vasculature. Those of ordinary skill in the art understandthat fish-scaling of stent structural elements may cause the stent toimpinge or snag upon the anatomical tissue either during endoluminaldelivery or after implantation. The term “unibody” as used herein isintended to mean a stent that is fabricated without the use of welds andas an integral body of material.

The inventive endoluminal stent may be, but is not necessarily,fabricated by vapor deposition techniques. Vapor deposition fabricationof the inventive stents offers many advantages, including, withoutlimitation, the ability to fabricate stents of complex geometries, theability to control fatigue life, corrosion resistance, corrosionfatigue, bulk and surface material properties, and the ability to varythe transverse profiles, Z-axis thickness and X-Y-axis surface area ofthe stent's structural elements in manners that affect the longitudinalflexibility, hoop strength of the stent and radial expansion profiles.

SUMMARY OF THE INVENTION

Endoluminal stent, covered stent and stent-graft design inherentlyattempts to optimize the functional aspects of radial expandability,i.e., the ratio of delivery diameter to expanded diameter, hoopstrength, longitudinal flexibility, longitudinal foreshorteningcharacteristics, column strength, fish-scaling of individual structuralmembers of the stent, fatigue life, corrosion resistance, corrosionfatigue, hemodynamics, biocompatibility and the capability ofstent-through-stent delivery. Conventional stent designs have had tocompromise one or more functional features of a stent in order tomaximize a particular functionality, e.g., longitudinal flexibility isminimized in order to achieve desirable column strength or high hoopstrengths are achieved at the expense of small ratios of radialexpandability. It is an objective of the present invention to providedesigns for endoluminal unibody stents that achieve balances between theratio of radial expandability, hoop strength, longitudinal flexibilityand column strength, with biocompatibility, hemodynamics, radioopacity,minimal or no fish-scaling and increased capacity forendothelialization.

In accordance with a preferred embodiment of the present invention, theinventive endoluminal stent is formed of a single piece of biocompatiblemetal or pseudometal and having a plurality of circumferential expansionmembers co-axially aligned along a longitudinal axis of the stent and aplurality of interconnecting members interconnecting adjacent pairs ofcircumferential expansion members. Each of the plurality ofcircumferential expansion members comprises a generally sinusoidal ringstructure having successive peaks and valleys interconnected by stentstrut members. Each of the interconnecting members interconnectsadjacent pairs of circumferential expansion members at approximatemid-points of stent strut members on the adjacent pairs ofcircumferential expansion members. In order to enhance longitudinalflexibility of the inventive stent, it has been found desirable toinclude minor terminal regions of each interconnecting member that arenarrower in width than a major intermediate region of theinterconnecting member. The minor terminal regions are positioned atboth the proximal and distal end of each interconnecting member and arenarrower in width to enhance flexion at the junction region between thestent strut member and the interconnecting member. Additionally, it hasbeen found desirable to form each of the minor terminal regions of theinterconnecting members in the form of generally C-shaped sectionsextending proximally or distally from the intermediate region of eachinterconnecting member.

In accordance with all embodiments of the present invention, each of theplurality of circumferential expansion members and the plurality ofinterconnecting members may be fabricated of like biocompatiblematerials, preferably, biocompatible metals or metal alloys. In thismanner, both the plurality of circumferential expansion elements and theplurality of interconnecting members have like physical materialproperties, e.g., tensile strength, modulus of elasticity, plasticdeformability, spring bias, shape memory or super-elastic properties.Alternatively, the plurality of circumferential expansion members andinterconnecting members may be fabricated of biocompatible materials,preferably, biocompatible metals or metal alloys which exhibit differentphysical or material properties. In this latter case, the plurality ofcircumferential expansion elements may, for example, be fabricated of aplastically deformable material, such as stainless steel, while theplurality of interconnecting members are fabricated of a shape memory orsuper-elastic material, such as nickel-titanium alloys, or of a springbiased material, such as stainless steel.

Heretofore, joints between discrete sections of endoluminal stentsrequired welds in order to join sections of the stent. One particularadvantage of the present invention is that by forming the stent usingvapor deposition techniques, not only are discrete sections atomicallyjoined without the use of welds, but different materials may be employedin different and discrete sections of the stent in order to impartdistinct material properties and, therefore, functionality, to thediscrete sections.

Finally, the present invention also includes a self-supportingendoluminal graft. As used herein the term “graft” is intended toindicate any type of tubular member that exhibits integral columnar andcircumferential strength and which has openings that pass through thethickness of the tubular member. The inventive self-supportingendoluminal graft preferably consists of a member formed of at least oneof a plurality of layers, each layer being comprised of a plurality offirst and interconnecting members which intersect one another, asdescribed above, to define a plurality of open regions betweenintersecting pairs of the first and interconnecting members. A webregion subtends at least a portion of the open region to at leastpartially enclose each of the plurality of open regions. Successiveadjacent layers of the plurality of layers are positioned such that theopen regions are staggered in the Z-axis transverse through the wall ofthe self-supporting endoluminal graft. By staggering the open regions,interlamellar spaces are created to facilitate endothelialization of theendoluminal graft.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a perspective view of an endoluminal stent in its expandeddiameter in accordance with the present invention.

FIG. 2 is a plan view of a first embodiment of the inventive endoluminalstent.

FIG. 3 is a plan view of a second embodiment of the inventiveendoluminal stent.

FIG. 4 is a plan view of a third embodiment of the inventive endoluminalstent.

FIG. 5 is a plan view of a fourth embodiment of the inventiveendoluminal stent.

FIG. 6 is a photomicrograph of an interconnecting member and portions ofcircumferential expansion members of the inventive endoluminal stent.

FIG. 7 is a photomicrograph depicting the inventive endoluminal stent inits constricted diameter for endoluminal delivery within a constrainingsheath.

FIG. 8 is a photomicrograph depicting the inventive endoluminal stentpartially released from a constraining sheath and radially expanding.

FIG. 9 is photomicrograph depicting the inventive endoluminal stent inits radially enlarged diameter.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In accordance with the present invention there is provided severalpreferred embodiments. In each of the preferred embodiments of thepresent invention, the general configuration of the inventiveendoluminal stent is substantially the same. Specifically and withparticular reference to FIG. 1, the inventive endoluminal stent 10consists generally of a tubular cylindrical element comprised of aplurality of circumferential expansion elements 12 generally formingclosed rings about the circumferential axis C′ of the stent 10 andarrayed in spaced apart relationship relative to one another coaxiallyalong the longitudinal axis L′ of stent 10. A plurality ofinterconnecting members 14 interconnects adjacent pairs of the pluralityof circumferential expansion elements 12. Each of the plurality ofcircumferential expansion elements 12 have a generally sinusoidalconfiguration with a plurality of peaks 12 p and a plurality of troughs12 t of each circumferential expansion member and a plurality of struts16 interconnecting adjacent peaks 12 p and troughs 12 t. The pluralityof peaks 12 p and the plurality of troughs 12 t in one circumferentialring member 12 may either be in phase or out of phase with the pluralityof peaks 12 p and troughs 12 t in adjacent circumferential ring members12. Additionally, within each circumferential ring member 12, the peaks12 p and troughs 12 t may have either regular or irregular periodicityor each of the plurality of circumferential expansion elements may haveregions of regular periodicity and regions of irregular periodicity.Each of the plurality of interconnecting members 14 preferably comprisegenerally linear elements having a width W_(i) that interconnect a strut16 of a first circumferential expansion element 12 with a strut 16 of asecond, adjacent circumferential element 12. Each of the plurality ofinterconnecting members has a generally rectangular transversecross-sectional shape. In accordance with each preferred embodiment ofthe present invention, the interconnection between each of the pluralityof interconnecting members 14 and the struts 16 occurs at an approximatemid-point along the length of the strut 16. Each of the plurality ofstruts 16 has a width W_(s) and is generally rectangular in transversecross-section.

Additionally, a plurality of terminal flange members 11, shown inphantom, may be provided in order to provide affixation points formounting a graft covering (not shown) onto the stent 10. The terminalflange members 11 may be positioned at the distal end, the proximal endor both ends of the stent 10 and preferably are formed generally linearprojections from either peak 12 p or a trough 12 t of a terminalcircumferential expansion element 12 at either or both of the proximalor distal ends of the stent 10. Each of the plurality of flange members11 may further include a rounded distal or proximal end region tofacilitate affixation of a graft covering.

With reference to FIGS. 2 and 6, to facilitate crimping the inventivestent 10 to its first, smaller delivery diameter, it has been foundpreferable to provide at each peak 12 p and trough 12 t a generallyU-shaped hinge element 22 that connects adjacent struts along eachcircumferential expansion member 12. In accordance with the preferredembodiments of the invention, it is desirable that each generallyU-shaped element hinge has a width W_(h) that is less than W_(s) of thestruts 16 to which it is connected. By making W_(h) less than W_(s), ithas been found that a greater degree of compression of the angle αformed between adjacent struts 16 interconnected by the generallyU-shaped hinge element 22 may be achieved, thereby lending a greaterdegree of compressibility to the inventive stent 10 than that foundwhere the U-shaped hinge element 22 was not employed.

Additionally, it has been found desirable, in accordance with the bestmode for the present invention, to provide strain-relief sections 18 and20 at opposing ends of each of the plurality of interconnecting members14. The strain-relief sections 18 and 20 comprise terminal sections ofthe interconnecting member 14 and have a width W_(t) that is less thanthe width W_(i) of the interconnecting member 14. In accordance with oneembodiment of the present invention, the strain-relief sections 18 and20 each have a generally C-shaped configuration and traverse a radius inconnecting the interconnection member 14 with the struts 16 of adjacentcircumferential expansion members 12. Alternate geometric configurationsof the C-shaped terminal strain-relief sections 18 and 20 are alsocontemplated by the present invention, such as S-shaped, V-shaped,M-shaped, W-shaped, U-shaped, or merely generally I-shaped extensionsprojecting co-axially along the longitudinal axis of eachinterconnecting member 14.

FIGS. 2–5 depict alternative preferred embodiments of the stent 10 ofthe present invention. Each of the preferred embodiments depicted inFIGS. 2–5 include the same circumferential expansion elements 12, eachhaving a plurality of peaks 12 p and troughs 12 t and formed of aplurality of struts 16 interconnected at the peaks 12 p and troughs 12t, and the generally U-shaped elements 22 forming the peaks 12 p andtroughs 12 t, with adjacent pairs of circumferential expansion elements12 being interconnected by the plurality of interconnecting members 14.Thus, in each of FIGS. 2–5, like elements are identified by likereference numerals. The alternative preferred embodiments of theinventive stent 30, 40, 50 and 60 illustrate in each of FIGS. 2, 3, 4and 5, respectively, differ principally in the position and orientationof the plurality of interconnecting members 14. In FIGS. 2–5, each ofthe stents 30, 40, 50 and 60 are illustrated in planar views. Thoseskilled in the art will understand that the planar view is depicted forease of illustration and that the stents depicted are tubular with linesA—A and B—B forming division lines along the longitudinal axis L′ of thestents in order to illustrate the stent geometry in a planar view.

In FIG. 2, stent 30 is comprised of a plurality of circumferentialexpansion members 12 and a plurality of interconnecting members 14. Eachof the plurality of interconnecting members 14 joins adjacent pairs ofcircumferential expansion members 12. Each interconnecting member 14forms a junction with a strut 16 of each of the adjacent circumferentialexpansion members 12 and intersects the strut 16 at approximately amid-point along the length of each strut 16. The plurality ofinterconnecting members 14 form groupings 14 a, 14 b, 14 c, 14 d, 14 eand 14 f along the longitudinal axis L′ of the stent 30. Because theinterconnecting members 14 lie in the folding planes of the peaks 12 pand troughs 12 t and struts 16 about angle α, it has been founddesirable to offset each of the interconnecting members 14 from a lineparallel to the longitudinal axis L′ of the stent 30 by an angle β inorder to enhance the folding properties of the circumferential expansionmembers 12 from a larger diameter to a smaller diameter of the stent 30.In stent 30, each of the plurality of interconnecting members 14 ingroupings 14 a–14 f have the same offset angle β and all of theplurality of interconnecting members 14 are parallel to each other. Inorder to accommodate the offset angle β, and provide for folding of theinterconnecting members 14 during compression of the stent 30 from itslarger diameter to its smaller diameter, the strain relief sections 18and 20 at terminal ends of each interconnecting member 14 have opposingorientations. Thus, when stent 30 is viewed in its tubular configurationfrom a proximal end view P, first strain relief section 18 has agenerally C-shaped configuration that has a right-handed or clockwiseorientation, while the second strain relief section 20, also having agenerally C-shaped configuration has a generally left-handed orcounterclockwise orientation.

In accordance with the preferred embodiment for stent 30, it has beenfound desirable to employ a 2:1 ratio of peaks l2 p or troughs 12 t tointerconnecting members. Thus, as depicted, there are six peaks 12 p andsix troughs 12 t in each of the plurality of circumferential expansionelements 12 and three interconnecting members 14 interconnect each pairof adjacent circumferential expansion elements 12. Similarly, betweenadjacent pairs of circumferential expansion elements 12, theinterconnecting members 14 are circumferentially offset one peak 12 pand one trough 12 t from the interconnecting members 14 in an adjacentpair of circumferential expansion elements 12. Thus, interconnectingelements in groups 14 a, 14 c and 14 e interconnect circumferentialexpansion element pairs 12 a–12 b, 12 c–12 d, 12 e–12 f, 12 g–12 h and12 i–12 j, interconnecting elements in groups 14 b, 14 d and 14 finterconnect circumferential expansion element pairs 12 b–12 c, 12 d–12e, 12 f–12 g, 12 h–12 i. With the interconnecting elements in group 14a, 14 c and 14 e each being offset by one peak l2 p and one trough 12 talong the circumferential axis of each circumferential expansion element12.

Turning to FIG. 3, stent 40 is illustrated and has a substantiallyidentical configuration of circumferential expansion elements 12 andinterconnecting elements 14, except that instead of employing a 2:1ratio of peaks 12 p or troughs 12 t to interconnecting elements, stent40 employs a 3:1 ratio, such that each circumferential expansion element12 a–12 i has six peaks 12 p and six troughs 12 t, but adjacent pairs ofcircumferential elements 12 are interconnected by only twointerconnecting elements 14. Like stent 30, the interconnecting elementsof a first circumferential expansion element pair are circumferentiallyoffset from the interconnecting elements of a second adjacentcircumferential expansion element pair, except in stent 40, the offsetis either one peak 12 p and two troughs 12 t or two peaks 12 p and onetrough 12 t. In stent 40 there are four groups of interconnectingelements 14 a, 14 b, 14 c and 14 d that interconnect the plurality ofcircumferential expansion elements 12. Interconnecting element groups 14a and 14 c interconnects circumferential expansion element pairs 12 b–12c, 12 d–12 e, 12 f–12 g and 12 h–12 i, and interconnecting elementgroups 14 b and 14 d interconnect circumferential expansion elementpairs 12 a–12 b, 12 c–12 d, 12 e–12 f and 12 g–12 h.

In stent 40, each of the interconnecting elements 14 are also angularlyoffset from the longitudinal axis of the stent by an angle β, exceptthat the plurality of interconnecting elements 14 are not all parallelrelative to each other. Rather, the interconnecting elements ininterconnecting element groups 14 a and 14 c are parallel to each otherand the interconnecting elements in interconnecting elements groups 14 band 14 d are parallel to each other, with the interconnecting elementsin groups 14 a and 14 c being offset from the longitudinal axis of thestent by an angle β− which is alternate to the angle β, also denotedangle β+, forming the offset from the longitudinal axis L′ for theinterconnecting elements in groups 14 b and 14 d. The designation angleβ+ and angle β− is intended to denote that these angles represent thesubstantially the same angular offset from the longitudinal axis L′, buthave alternate orientations relative to the circumferential axis of thestent 40.

Turning now to FIG. 4 in which stent 50 is depicted. Like stents 30 and40 described above, stent 50 shares the common elements ofcircumferential expansion elements 12, having a plurality of peaks 12 pand troughs 12 t interconnecting a plurality of struts 16, and U-shapedsections 22, and interconnecting elements 14. In stent 50, however, theplurality of interconnecting elements 14 form two groups ofinterconnecting elements 14 a and interconnecting elements 14 b. Each ofthe individual interconnecting elements 14 in interconnecting elementgroups 14 a and 14 b are also angularly offset from the longitudinalaxis L′ of the stent 50 by angle β. Moreover, within each pair ofadjacent circumferential expansion elements 12, the interconnectingelement groups 14 a and 14 b are circumferentially offset from eachother by three peaks 12 p and three troughs 12 t. Within each group ofinterconnecting elements 14 a and 14 b, however, each of the pluralityof individual interconnecting elements 14 are generally aligned along acommon longitudinal axis. In this manner, with the exception of the mostproximal 12 a and the most distal 12 i circumferential ring elements,each of the plurality of interconnecting elements form a substantiallyfour-point junction 19 at approximately a mid-point a strut 16 on eachof circumferential expansion elements 12 b–12 h. The substantiallyfour-point junction 19 is formed between a distal strain relief section20 of one interconnecting member with a proximal side of a strut 16 anda proximal strain relief section 18 of an adjacent interconnectingelement 14 with a distal side of the same strut 16.

Finally, turning to FIG. 5, there is illustrated stent 60 which, likestents 30, 40 and 50 is comprised of a plurality of circumferentialexpansion elements 12 and interconnecting elements 14 that interconnectadjacent pairs of circumferential expansion elements 12. Like stent 40of FIG. 3, stent 60 has groupings of interconnecting elements 14 intointerconnecting element groups 14 a, 14 b, 14 c and 14 d. In stent 60,however, interconnecting element groups 14 a and 14 d interconnectidentical pairs of circumferential expansion elements 12 andinterconnecting element groups 14 b and 14 c interconnect identicalpairs of circumferential expansion elements 12. Each of theinterconnecting elements in interconnecting element groups 14 b and 14 dare angularly offset from the longitudinal axis L′ of the stent 60 by anangle β− and are parallel to one and other. Similarly, each of theinterconnecting elements in interconnecting element groups 14 a and 14 care angularly offset from the longitudinal axis L′ of the stent 60 by anangle β+ and are parallel to one and other.

For each adjacent pair of circumferential expansion elements 12, theinterconnecting elements 14 have different orientations of angularoffset from the longitudinal axis L′ of the stent 50. For example, forcircumferential expansion element pair 12 a–12 b, the interconnectingelements of group 14 b and group 14 c are offset by angle β− and byangle β+, respectively. In the adjacent circumferential expansionelement pair 12 b–12 c, the interconnecting elements of group 14 a and14 d are offset by angle β+ and by angle β−, respectively. Thus, betweenadjacent pairs of circumferential elements 12, the interconnectingelements are out of phase, in that they have different angularorientations of angle β. Additionally, between adjacent pairs ofcircumferential elements 12, the interconnecting elements arecircumferentially offset by a single peak 12 p, with interconnectingelement group 14 a being circumferentially offset from interconnectingelement group by a single peak 12 p, and interconnecting element group14 c being circumferentially offset from interconnecting element group14 d by a single peak 12 p. Furthermore, there are differentcircumferential offsets between interconnecting element group pairs 14b–14 c and 14 a–14 d within individual pairs of adjacent circumferentialexpansion elements 12. The circumferential offset betweeninterconnecting element group pair 14 b–14 c is two peaks 12 p and threetroughs 12 t, while the circumferential offset between interconnectingelement group pair 14 a–14 d is four peaks 12 p and three troughs 12 t.

Those skilled in the art will appreciate that the foregoing embodimentof stents 10, 20, 30, 40 and 50 describe various geometries allcomprised of common structural elements, namely, circumferentialexpansion elements 12 having a plurality of peaks 12 p and troughs 12 tand struts 16 interconnected by hinge elements 22. Furthermore, thoseskilled in the art will understand that variations on the number of andpositioning of the interconnecting members 14 between adjacent pairs ofcircumferential expansion elements 12 and along the circumferential axisof the stent are also contemplated by the present invention and that thespecific embodiments illustrated and described with reference to thefigures is exemplary in nature.

FIG. 3, however, represents a particularly preferred embodiment of theinventive stent 40. Inventive stent 40 was fabricated by laser-cuttingthe described geometry from a nickel-titanium hypotube. After lasercutting, the stent 40 was annealed to set shape memory properties forthe stent 40 with a fully expanded, enlarged outer diameter of 5.8 mmand a length of 30.6 mm. Stent 40 was capable of being crimped to asmaller, crimped outer diameter of 1.4 mm and was placed within aconstraining sheath as illustrated in FIG. 7. Stent 40 exhibitedexcellent crimpability with the struts 16 folding at the generallyU-shaped hinge elements 22 through angle α without appreciableinterference between the circumferential expansion elements 12 and theinterconnecting elements 14.

During radial expansion of the stent 40 from its first constrainedsmaller diameter, i.e., 1.4 mm, to its second enlarged radially expandeddiameter, i.e., 5.8 mm, the stent 40 exhibited no foreshorteningcharacteristic of many stent geometries known in the art. In contrast toforeshortening the stent 40 unexpectedly elongated by 2.5%. Heretofore astent that elongates upon radial expansion is unknown in the art.

FIG. 8 depicts stent 40 radially expanding as it the constraining sheathis being withdrawn from the stent 40. FIG. 9 depicts stent 40 invirtually its fully radially expanded enlarged diameter, with just aproximal section of the stent 40 be constrained in the constrainingsheath (not pictured). FIG. 6 is an enlarged section of the stent 40illustrating the mid-strut connection between the circumferentialexpansion element 12 and the interconnecting element 14 at the proximaland distal strain relief sections 18 and 20, and clearly showing thegenerally U-shaped hinge elements 22, the peaks 12 p and troughs 12 t ofeach circumferential expansion element 12. FIG. 6 also clearly depictsthe differences in the widths W_(t) of the proximal and distal strainrelief sections and the width W₁ of the body of the interconnectingmember 14, as well as the difference between the width W_(h) of theU-shaped hinge element 22 and the width W_(s) of the strut 16.

The plurality of circumferential expansion elements 12 andinterconnecting members 14, and components sections thereof, arepreferably made of materials selected from the group consisting oftitanium, vanadium, aluminum, nickel, tantalum, zirconium, chromium,silver, gold, silicon, magnesium, niobium, scandium, platinum, cobalt,palladium, manganese, molybdenum and alloys thereof, and nitinol andstainless steel. The plurality of circumferential expansion elements 12and the plurality of interconnecting members 14 may be made of the samematerial or of different materials and have the same material propertiesor have different material properties. The term “material properties” isintended to encompass physical properties, including without limitation,elasticity, tensile strength, mechanical properties, hardness, bulkand/or surface grain size, grain composition, and grain boundary size,intra and inter-granular precipitates. Similarly, the materials selectedfor the plurality of circumferential expansion elements 12 and theplurality of interconnecting members 14 may be selected to have the sameor different chemical properties. The term “chemical properties” isintended to encompass both any chemical reaction and change of statethat the material may undergo after being implanted into a body and thephysiological response of the body to the material after implantation.

While the inventive stents may be fabricated by chemical, thermal ormechanical ablative methods known in the art, such as chemical etching,laser cutting, EDM or water jet processes, it is envisioned that apreferred method for fabricating the inventive stents is by physicalvapor deposition techniques. Physical vapor deposition techniques affordthe ability to tightly control both the tolerances of the stentgeometries as well as the physical and chemical properties of the stentand the stent materials. The inventive stents 10, 30, 40, 50 and 60,including each of their elements, namely the plurality ofcircumferential expansion elements 12 and interconnecting members 14 andcomponent sections thereof, are preferably made of a bulk materialhaving controlled heterogeneities on the luminal surface thereof. As isdescribed in co-pending, commonly assigned, U.S. patent application Ser.No. 09/754,304 filed Dec. 22, 2000, which is a divisional of U.S. Pat.No. 6,379,383 issued Apr. 30, 2002, which is hereby incorporated byreference, heterogeneities are controlled by fabricating the bulkmaterial of the stent to have defined grain sizes, chemical and intra-and intergranular precipitates and where the bulk and surface morphologydiffer, yielding areas or sites along the surface of the stent whilemaintaining acceptable or optimal protein binding capability. Thecharacteristically desirable properties of the inventive stent are: (a)optimum mechanical properties consistent with or exceeding regulatoryapproval criteria, (b) minimization of defects, such as cracking or pinhole defects, (c) a fatigue life of 400 MM cycles as measured bysimulated accelerated testing, (d) corrosion and/or corrosion-fatigueresistance, (e) biocompatibility without having biologically significantimpurities in the material, (f) a substantially non-frictional abluminalsurface to facilitate atraumatic vascular crossing and tracking andcompatible with transcatheter techniques for stent introduction, (g)radiopaque at selected sites and MRI compatible, (h) have a luminalsurface which is optimized for surface energy and microtopography, (i)minimal manufacturing and material cost consistent with achieving thedesired material properties, and (j) high process yields.

In accordance with the present invention, the foregoing properties areachieved by fabricating a stent by the same metal depositionmethodologies as are used and standard in the microelectronics andnano-fabrication vacuum coating arts, and which are hereby incorporatedby reference. The preferred deposition methodologies include ion-beamassisted evaporative deposition and sputtering techniques. In ionbeam-assisted evaporative deposition it is preferable to employ dual andsimultaneous thermal electron beam evaporation with simultaneous ionbombardment of the substrate using an inert gas, such as argon, xenon,nitrogen or neon. Bombardment with an inert gas, such as argon ionsserves to reduce void content by increasing the atomic packing densityin the deposited material during deposition. The reduced void content inthe deposited material allows the mechanical properties of thatdeposited material to be similar to the bulk material properties.Deposition rates up to 20 nm/sec are achievable using ion beam-assistedevaporative deposition techniques.

When sputtering techniques are employed, a 200-micron thick stainlesssteel film may be deposited within about four hours of deposition time.With the sputtering technique, it is preferable to employ a cylindricalsputtering target, a single circumferential source that concentricallysurrounds the substrate that is held in a coaxial position within thesource. Alternate deposition processes which may be employed to form thestent in accordance with the present invention are cathodic arc, laserablation, and direct ion beam deposition. When employing vacuumdeposition methodologies, the crystalline structure of the depositedfilm affects the mechanical properties of the deposited film. Thesemechanical properties of the deposited film may be modified bypost-process treatment, such as by, for example, annealing,high-pressure treatment or gas quenching.

Materials to make the inventive stents are chosen for theirbiocompatibility, mechanical properties, i.e., tensile strength, yieldstrength, and their ease of deposition include the following: elementaltitanium, vanadium, aluminum, nickel, tantalum, zirconium, chromium,silver, gold, silicon, magnesium, niobium, scandium, platinum, cobalt,palladium, manganese, molybdenum and alloys thereof, such aszirconium-titanium-tantalum alloys, nitinol, and stainless steel.

During deposition, the chamber pressure, the deposition pressure and thepartial pressure of the process gases are controlled to optimizedeposition of the desired species onto the substrate. As is known in themicroelectronic fabrication, nano-fabrication and vacuum coating arts,both the reactive and non-reactive gases are controlled and the inert ornon-reactive gaseous species introduced into the deposition chamber aretypically argon and nitrogen. The substrate may be either stationary ormoveable, either rotated about its longitudinal axis, or moved in an X-Yplane within the reactor to facilitate deposition or patterning of thedeposited material onto the substrate. The deposited material may bedeposited either as a uniform solid film onto the substrate, orpatterned by (a) imparting either a positive or negative pattern ontothe substrate, such as by etching or photolithography techniques appliedto the substrate surface to create a positive or negative image of thedesired pattern or (b) using a mask or set of masks which are eitherstationary or moveable relative to the substrate to define the patternapplied to the substrate. Patterning may be employed to achieve complexfinished geometries of the resultant stent, both in the context ofspatial orientation of the pattern as well as the material thickness atdifferent regions of the deposited film, such as by varying the wallthickness of the material over its length to thicken sections atproximal and distal ends of the stent to prevent flaring of the stentends upon radial expansion of the stent.

The stent may be removed from the substrate after stent formation by anyof a variety of methods. For example, the substrate may be removed bychemical means, such as etching or dissolution, by ablation, bymachining or by ultrasonic energy. Alternatively, a sacrificial layer ofa material, such as carbon or aluminum, may be deposited intermediatethe substrate and the stent and the sacrificial layer removed bymelting, chemical means, ablation, machining or other suitable means tofree the stent from the substrate.

The resulting stent may then be subjected to post-deposition processingto modify the crystalline structure, such as by annealing, or to modifythe surface topography, such as by etching to affect and control theheterogeneities on the blood flow surface of the stent.

A plurality of microgrooves may be imparted onto the luminal and/orabluminal surface of the stent 10, as is more fully described inInternational Publication No. WO 99/23977, published 20 May 1999, whichis commonly assigned with the present application and is herebyincorporated by reference. The plurality of microgrooves may be formedeither as a post-deposition process step, such as by etching, or duringdeposition, such as by depositing the stent-forming material onto amandrel which has a microtopography on the surface thereof which causesthe metal to deposit with the microgroove pattern as part of thedeposited material.

Each of the preferred embodiments of the present invention arepreferably fabricated by employing a vapor deposition technique whichentails vapor depositing a stent-forming metal onto a substrate. Thesubstrate may be planar or cylindrical and is either pre-patterned withone of the preferred geometries of first and interconnecting members, ineither positive or negative image, or the substrate may be un-patterned.Where the substrate is un-patterned, the deposited stent-forming metalis subjected to post-deposition patterning to pattern the depositedstent-forming metal into one of the preferred geometries of the firstand interconnecting members. In all embodiments of the present inventionfabricated by vapor deposition techniques, the need for post-depositionprocessing of the patterned endoluminal stent, e.g., modifying thesurface of the stent by mechanical, electrical, thermal or chemicalmachining or polishing, is eliminated or minimized.

Vapor deposition fabrication of the inventive endoluminal stents offersmany advantages, including, for example, the ability to fabricate stentsof complex geometries, ultrafine dimensional tolerances on the order ofAngstroms, the ability to control fatigue life, corrosion resistance,corrosion fatigue, inter- and intra-granular precipitates and theireffect on corrosion resistance and corrosion fatigue, bulk materialcomposition, bulk and surface material properties, radioopacity, and theability to vary the transverse profiles, Z-axis thickness and X-Y-axissurface area of the stent structural elements in manners that affect thelongitudinal flexibility, hoop strength, and radial expansion behaviorand profile of the stent. Bulk material composition may be adjusted toemploy elemental fractions in alloy compositions that are not feasiblewhen using conventionally formed metals. This results in achieving theability to tailor the alloy compositions in a manner that optimizes thealloy composition for a desired material or mechanical property. Forexample, nickel-titanium tubes exhibiting shape memory and/orsuperelastic properties were made employing in excess of 51.5 atomicpercent nickel, which is not achievable using conventional workingtechniques due to high plateau stresses exhibited by the material.Specifically, the present inventors have fabricated nickel-titaniumalloy tubes employing the method of the present invention that containbetween 51.5 and 55 atomic percent nickel.

Vapor deposition of the inventive endoluminal stent, in accordance witha preferred embodiment of the present invention, significantly reducesor virtually eliminates inter- and intra-granular precipitates in thebulk material. It is common practice in the nickel-titanium endoluminaldevice industry to control transition temperatures and resultingmechanical properties by altering local granular nickel-titanium ratiosby precipitation regimens. In the present invention, the need to controlprecipitates for mechanical properties is eliminated. Wherenickel-titanium is employed as the stent-forming metal in the presentinvention, local nickel-titanium ratios will be the same or virtuallyidentical to the nickel-titanium ratios in the bulk material, whilestill allowing for optimal morphology and eliminating the need foremploying precipitation heat treatment. The resulting depositedstent-forming metal exhibits superior corrosion resistance, and hence,resistance to corrosion fatigue, when compared to conventional wroughtnickel-titanium alloys.

The plurality of circumferential expansion elements 12 and the pluralityof interconnecting members 14 may be conformationally configured duringvapor deposition to impart a generally rectangular, ovular or ellipticaltransverse cross-sectional profile with either right angled edges orwith chamfered or curved leading and trailing luminal and abluminalsurface edges in the longitudinal axis of the stent in order to providebetter blood flow surface profiles.

While the present inventions have been described with reference to theirpreferred embodiments, those of ordinary skill in the art willunderstand and appreciate that a multitude of variations on theforegoing embodiments are possible and within the skill of one ofordinary skill in the vapor deposition and stent fabrication arts, andthat the above-described embodiments are illustrative only and are notlimiting the scope of the present invention which is limited only by theclaims appended hereto.

1. An endoluminal stent comprising: a. a plurality of circumferentialexpansion elements co-axially spaced to form a tubular configuration andeach having an undulating pattern of peaks and valleys interconnected bystruts, wherein the struts form linear sections and are interconnectedat the peaks and valleys by hinge elements; and b. a plurality of linearinterconnecting elements interconnecting adjacent pairs ofcircumferential expansion elements and joined at approximate mid-pointsof adjacent struts along a longitudinal axis of the endoluminal stent,the interconnecting elements having curvilinear first and secondterminal sections, each section of the first and second terminal sectionhaving a narrower width than a width of the interconnecting elements. 2.The endoluminal stent according to claim 1, wherein each of theplurality of circumferential expansion elements further comprises azig-zag configuration along a circumferential axis of the endoluminalstent wherein the struts are uniform in width throughout the entiresection of the struts.
 3. The endoluminal stent according to claim 1,wherein the curvilinear first and second terminal sections arepositioned at opposing ends of each interconnecting element that joinwith the struts.
 4. The endoluminal stent according to claim 3, whereineach of the plurality of circumferential expansion elements are integraland monolithic with each of the plurality of interconnecting elements.5. The endoluminal stent according to claim 4, wherein the curvilinearfirst and second terminal sections of the plurality of linearinterconnecting elements further comprise C-shaped sections.
 6. Theendoluminal stent according to claim 1, wherein the plurality of linearinterconnecting elements are all parallel to each other.
 7. Theendoluminal stent according to claim 1, wherein the plurality of linearinterconnecting elements are arrayed as at least two groups ofinterconnecting elements along a longitudinal axis of the endoluminalstent, a first of the at least two groups having a different angularorientation relative to the longitudinal axis of the endoluminal stentthan a second of the at least two groups.
 8. The endoluminal stentaccording to claim 1, wherein the endoluminal stent elongates along thelongitudinal axis of the endoluminal stent as it expands from a smallerdiameter to a larger diameter.
 9. An endoluminal stent comprising: a. aplurality of circumferential expansion elements co-axially spaced toform a generally tubular configuration, each having a generallyundulating pattern of peaks and valleys interconnected by struts forminga generally zig-zag configuration along a circumferential axis of theendoluminal stent wherein the struts form generally linear sections andare interconnected at the peaks and valleys by hinge elements having awidth narrower than a width of the struts; and b. a plurality ofgenerally linear interconnecting elements comprising generallycurvilinear first and second terminal sections at opposing ends of eachinterconnecting element that join with the struts, each of the generallycurvilinear first and second terminal sections of the plurality ofgenerally linear interconnecting elements further comprising generallyC-shaped sections having a width narrower than a width of the remainderof the interconnecting element, the plurality of generally linearinterconnecting elements interconnecting adjacent pairs ofcircumferential expansion elements and joined at approximate mid-pointsof adjacent struts along a longitudinal axis of the endoluminal stent;and wherein each of the plurality of circumferential expansion elementsare integral and monolithic with each of the plurality ofinterconnecting elements.
 10. An endoluminal stent comprising: a. aplurality of circumferential expansion elements co-axially spaced toform a tubular configuration and each having an undulating pattern ofpeaks and valleys interconnected by struts; and b. a plurality of linearinterconnecting elements interconnecting adjacent pairs ofcircumferential expansion elements and joined by strain relief sectionsat approximate mid-points of adjacent struts along a longitudinal axisof the endoluminal stent; c. wherein the strain relief sections have awidth narrower than a width of the interconnecting elements.